This invention relates to thermal protective compositions which form chars when exposed to fire or other thermal extremes. The invention is particularly well suited to use in epoxy-based intumescent coatings for substrates, but its usefulness is not limited thereto.
The situations in which it is desirable to protect materials from heat and flame include, for example, protecting static structures such as petroleum storage tanks, chemical production equipment, electrical cable trays, and structural steel from fire; protecting transportation equipment such as tank cars, aircraft cabins and seat cushions from the same risks; protecting the exterior surfaces of spacecraft and high performance aircraft from heat generated by atmospheric friction; and protecting the nozzles of rocket engines from the heat of propellant gases.
Various compositions are known which provide protection against fire and other thermal extremes, such as temperatures above about 300.degree. C. Some of the compositions are foamed inorganic passive insulative compositions which protect merely by their low thermal conductivity and their thickness as applied. These include, for example, foamed cement or intumesced silicates. The present invention is not concerned with such systems, but with systems which include a polymeric binder and which form a char when exposed to fire or hyperthermal conditions. The char-forming compositions may operate by various modalities. The compositions may be used in various forms, including thick film (mastic) coatings, thin film coatings, castings, extrusions, and others. The compositions may include organic or inorganic binders and various additives. Upon exposure to heat the compositions slowly lose weight as portions of the composition are volatilized, and a char is formed which provides a measure of protection against the transfer of heat energy. Eventually, the char is consumed by physical erosion and by chemical processes, primarily oxidation by oxygen in the air and by free radicals produced by the coating or otherwise in a fire environment, and protection is lost. The length of time required for a given temperature rise across a predetermined thickness of the composition, under specified heat flux, environmental, and temperature conditions, is a measure of the effectiveness of the composition in providing thermal protection.
When subjected to fire or other hyperthermal conditions, different coatings behave differently.
Ablative coatings swell to less than twice their original thickness. They provide limited passive thermal protection, but they tend to produce dense chars having good physical and chemical resistance.
Intumescent coatings swell to produce a char more than five times the original thickness of the coating. This char provides an insulative blanket which provides superior thermal efficiency, but at the cost of some of the physical and chemical properties of the ablative coatings. The char of the intumescent materials tends to form coarse and irregular cell structures, cracks, and fissures as it expands, and the char may not expand uniformly at corners, leaving areas where the char provides far less protection than the average thermal protection of the underlying structure. Examples of the intumescent systems include silicate solutions or ammonium phosphate paints or mastic compositions such as those disclosed in Nielsen et al., U.S. Pat. No. 2,680,077, Kaplan, U.S. Pat. No. 3,284,216, or Ward et al., U.S. Pat. No. 4,529,467.
A third type of char-forming coating is disclosed in Feldman, U.S. Pat. No. 3,849,178. When subjected to thermal extremes, these compositions both undergo an endothermic phase change and expand two to five times their original thickness to form a continuous porosity matrix. These coatings tend to be tougher than intumescent coatings. They provide far longer thermal protection than ablative coatings, frequently longer than intumescent coatings, in part because the gasses formed by the endothermic phase change provide active cooling as they work their way through the open-cell matrix. These coatings may also have a tendency to crack and form voids and fissures.
The present invention relates primarily to the second and third types of systems. In its broader aspects, however, it is also applicable to ablative char-forming coatings. It is not, however, intended for use in elastomeric coatings, such as silicone rubber coatings.
Various methods and structures have also been used or proposed for applying these thermal protective coating materials. The most frequent approach is to apply the materials directly to the substrate, without additional structure. For many applications, however, a reinforcing material, such as fiberglass sheet or a wire mesh, has been embedded in the coating material to strengthen the material and prevent it from cracking or falling off the substrate under conditions of flame or thermal extreme. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Billing et al, U.S. Pat. No. 3,913,290, Kaplan, U.S. Pat. No. 3,915,777, and Billing et al, U.S. Pat. No. 4,069,075. Sometimes the materials are first applied to a reinforcing structure such as a flexible tape or flexible wire mesh, and the combined structure is applied to the substrate. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Pedlow, U.S. Pat. No. 4,018,962, Peterson et al, U.S. Pat. No. 4,064,359, Castle, U.S. Pat. No. 4,276,332, and Fryer et al, U.S. Pat. No. 4,292,358. In these last-mentioned systems, the purpose of the reinforcing structure may be both to strengthen the resulting composite and to permit its application to a substrate without directly spraying, troweling or painting the uncured coating materials onto the substrate. In any of the foregoing methods and structures, multiple layers are frequently applied to the substrate to provide additional protection.
Presently known materials and methods, however, are not as efficient, in terms of length of protection for a given weight of protective material, as desirable. Efficiency is particularly important because in many applications weight or volume is critically limited. Moreover, heavily loading coating materials with fire retardants may seriously impair their physical characteristics and otherwise limit their suitability as coatings, for example by limiting their film-forming characteristics or their water-resisting characteristics. Presently known materials are thus frequently limited to certain types of applications.